An experiment was conducted at Umudike in 1981 and 1982 on a sandy loam soil in the rain forest zone of Nigeria, to determine the critical weed‐free requirement and the critical duration of weed interference in cocoyam/sweet potato/maize intercrop combination. The major weeds present in the weedy control crop mixture during the 2 years included grasses, broad‐leaves and sedges. The grasses were mainlyCynodon dactylon, Digitaria horizontalis, Eleusine indica, Panicum maximumandPaspalum scrobiculatum.The dominant broadleaved weeds includedCommelina benghalensis, lpomoea involucrata, Tridax procumbens, Mimosa pudica, Icacina trichantha, Vernonia cinerea, Calapogonium mucunoidesandChromolaena odorata.The major sedges comprised,Cyperus esculentus, Cyperus distans, Fimbristylis barteriandMariscus alternifollus.Uncontrolled weed growth caused a 2 year average yield loss of?1731.00 (? = Nigerian shillings) or 76% in cocoyam/sweet potato/maize intercrop combination compared with the yield (? 2260.00) of the mixture weeded every 28 days after planting. Intercropping the three crops significantly out‐yielded any of the sole crop components on per unit area basis. The yield of the crop mixture was most seriously impaired by weed interference during the first 28 days after planting but also caused serious yield reduction in the crop mixture if not removed during the first 56 days after planting.